How and why organisms are distributed as they are has long intrigued evolutionary biologists1, 2, 3, 4. The tendency for species to retain their ancestral ecology has been demonstrated in distributions on local and regional scales5, 6, 7, but the extent of ecological conservatism over tens of millions of years and across continents has not been assessed8, 9, 10, 11, 12, 13. Here we show that biome stasis at speciation has outweighed biome shifts by a ratio of more than 25:1, by inferring ancestral biomes for an ecologically diverse sample of more than 11,000 plant species from around the Southern Hemisphere. Stasis was also prevalent in transocean colonizations. Availability of a suitable biome could have substantially influenced which lineages establish on more than one landmass, in addition to the influence of the rarity of the dispersal events themselves. Conversely, the taxonomic composition of biomes has probably been strongly influenced by the rarity of species' transitions between biomes. This study has implications for the future because if clades have inherently limited capacity to shift biomes13, then their evolutionary potential could be strongly compromised by biome contraction as climate changes.